1. Field of The Invention
This invention relates to an angle of rotation detector which detects an angle of rotation using a resolver with one rotor and two stators.
2. Related Art
Heretofore, resolvers used to detect an angle of rotation were equipped with two stators, shown as 13 and 14 in FIG. 5. Two sine waves, phaseshifted relative to each other at an electrical angle of 90.degree., as shown in FIG. 6, are supplied to stators 13 and 14. This causes an excitation current to flow. The phase difference .THETA. between the output induced in a single rotor 15 and the excitation signal is obtained. This phase angle .THETA. is considered to be the angle of rotation. A conventional device for detecting angle of rotation using sine wave excitation is shown in FIG. 7. This device consists of: oscillator 21; dividing circuit 22 to accept pulses from oscillator 21 and divide them; circuit 23, which generates a signal to start the timing; sine wave generator circuit 24 and cosine generator circuit 25, which generate, respectively, a sine wave and a cosine wave synchronously with the signal from circuit 23; driver circuits 26 and 27; resolver 28; output circuit 29, which derives the rotor output of resolver 28; low pass filter 30, which eliminates harmonics components from the output; comparator 31, which detects the zero-crossing point; counter 32, which counts the pulse signals from divider circuit 22 from the time the start signal is received until the detection of the zero-crossing point causes the stop signal to be input; and CPU 33.
The conventional device just described to detect angle of rotation employs a sinusoidal excitation signal. It thus requires both a circuit to generate a sine wave and a circuit to generate a cosine wave. However, a circuit which can generate two such phase-shifted sine waves with good accuracy requires a large number of components, resulting in a relatively expensive device.
Employing an easier to generate non-sinusoidal excitation current source producing a non-sinusoidal wave, such as a rectangular wave, results in a large measurement error due to the effect of harmonics, which are contained within the rectangular wave. A low pass filter is generally used to eliminate such errors. However, cutting the tertiary harmonics sufficiently (by a voltage ratio of 40 dB), while allowing the basic or fundamental wave to pass, requires a multi-stage active filter or LC filter having a steep cutoff characteristic to effectively to break the circuit. This complicates the design. Temperature dependency considerations of the phase shift at the frequency of the fundamental wave necessitates a design which reduces the measurement accuracy of the angle (which is perhaps 0.5.degree.) within the applicable range of temperature (perhaps 0.degree. C. to 50.degree. C.). Thus, the filter must use elements whose response to temperature variations is extremely small, driving up the cost of the device.